A fluorescent lamp fixture requires a ballast for providing the starting and operating current to one or more fluorescent lamps. These ballasts utilize a transformer, having a magnetic core and a coil assembly. The magnetic core is usually laminated, i.e., made up of a number of thin ferromagnetic plates insulated from one another. The coil assembly usually includes a pair of coils, commonly referred to as a primary and a secondary winding, with each winding formed by winding a magnet wire on a spool or bobbin made from plastic or some other insulating material. The windings are disposed on a central leg of the magnetic core with each bobbin having a hollow center for fitting onto the central core leg. Two outside core legs are added to surround the windings to maximize flux concentration. During transformation, energy is transferred from the primary winding to the secondary winding by electromagnetic induction.
Ballast transformers may include one or more ferromagnetic shunts positioned between the primary and secondary windings. Such shunts increase the leakage reactance of the transformer by providing a flux leakage path between the primary and secondary windings. This flux leakage path is controlled by the air gap between the shunt and the core legs. The shunt, therefore, diverts a portion of the magnetic flux generated by the primary winding to prevent coupling with the flux generated by the secondary winding. Shunted transformers also limit the short-circuit current to a greater degree that those transformers that do not include such shunts, with the current reduction varying with the spacing (air gap) between the shunt and the adjacent core.
A fixed shunt method is typically used to incorporate a shunt in a ballast core. With this method, the shunt is integrally formed with the outside or inside legs of a ballast core, and, thus, by proper sizing maintains the appropriate air gap. However, this method is costly due to the large amount of scrap generated during stamping of the leg laminations which include the shunt portions.
Another method utilizes a U-shaped shunt, composed of a plurality of planar ferromagnetic laminations, which is inserted between the windings, with the upright legs of the shunt wrapped with tape to provide a snug fit between the center and end core legs. The shunt laminations may be held together by the tape, or may be bound with adhesives or fasteners. The thickness of the tape determines the gap between the shunt and the core, with compression of the tape maintaining the proper gap over the life of the ballast. This method avoids the large generation of scrap associated with the fixed shunt method.
A problem with this method for including separate shunts in ballasts is that with time, the tape may deteriorate or cold flow, allowing the shunt to shift position. Not only does this alter the short-circuit current, but in typical ballast transformers used for electrical lighting, the shunt may vibrate, causing a low-level hum which is considered a nuisance.
Another problem is proper positioning of the lower crossover portion of the U-shaped shunt relative to the central core leg. In high reactance type ballast assemblies, slight changes in positioning of the shunt cross over portion, on the order of 10-15 thousands, can have a major impact on performance characteristics. Consequently, shifting of the shunt has resulted in inconsistent performance, ballast to ballast, with increased failures during performance testing.